Properties of Nanostructured One-Dimensional and Composite Thermoelectric Materials
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Properties of
Nanostructured One-Dimensional and Composite Thermoelectric Materials
Apparao M. Rao, Xiaohua Ji, and Terry M.Tritt Abstract Over a decade ago, Dresselhaus predicted that low-dimensional systems would one day serve as a route to enhanced thermoelectric performance. In this article, recent results in the thermoelectric properties of nanowires and nanotubes are discussed. Various synthesis techniques will be presented, including chemical vapor deposition for the growth of thermoelectric nanostructures in templated alumina. Electrical transport measurements of carbon nanostructures, such as resistivity and thermopower, have revealed some very interesting thermoelectric properties. Challenges still remain concerning the measurement of individual nanostructures such as nanowires. Much work has been performed on the thermoelectric properties of carbon nanotubes, and these results will be highlighted. In addition, routes for enhanced thermoelectric materials have focused on incorporating nanostructures within the bulk materials. The role of these “hybrid composite structures” based on nanomaterials incorporated into the bulk matrix and the potential for enhanced performance are discussed. Keywords: composite, nanostructure, thermal conductivity, thermoelectricity.
Background The discovery and development of thermoelectric (TE) materials with a high figure of merit (ZT 1) has proven to be a challenging task in materials science and engineering. Worldwide research efforts have culminated in somewhat of a twopronged strategy for identifying this class of TE materials: engineered bulk materials and nanostructured materials. In the first approach, complex crystal structures have been designed and synthesized with the aim of attaining phononglass/electron-crystal characteristics (low lattice thermal conductivity as in glass, with high electrical conductivity as in
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metals) for high ZT. Recent progress on bulk thermoelectric materials is discussed in the article by Nolas et al. in this issue. The second approach is motivated by the presence of increased electron density of states at the Fermi level in nanostructured materials and the possibility of exploiting boundary scattering to reduce the thermal conductivity. As indicated by its prefix, a nanostructure has one of its crucial dimensions on the order of 1–100 nm. Various forms of nanostructures, such as nanotubes, nanowires, nanorods, nanoparticles, ultrathin films, quantum wells, and superlattices,
have been prepared in the laboratory and are providing valuable insights for engineering materials with improved TE properties. In this issue, Böttner et al. provide an overview of the progress achieved in 2D nanostructured TE materials, specifically, superlattices and quantum well materials. Nanocomposites that involve the incorporation of any of the aforementioned nanostructures in the corresponding bulk material are also gaining much interest, since enhanced TE properties are expected to result from interfacial reflection and scattering of phonons in th
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